1. Field of the Invention
This invention relates to an atmospheric pressure chemical vapor deposition (APCVD) reactor system and more particularly to an apparatus and method for removing deposits which accumulate within the APCVD system.
2. Background of the Relevant Art
Chemical vapor deposition (CVD) reactors are well known. CVD reactors are used to deposit thin films of various compositions upon a semiconductor substrate. CVD reactors can also be used to pre-deposit a dopant material upon the semiconductor substrate. After the dopant is placed, it can then be driven into the substrate using well known thermal diffusion techniques. CVD systems are therefore recognized as being capable of depositing a film upon the substrate or diffusing a dopant into the substrate. Accordingly, CVD systems are well adapted to many different applications and remain a mainstay in semiconductor fabrication.
There are many different types of CVD systems. Generally speaking a CVD system operates by placing a vaporized deposition or diffusion material into the reactor and over the target or semiconductor substrate. As the vaporized material passes over the substrate, it nucleates and adsorbs upon the substrate. The reaction chamber is typically heated from 300.degree. C. to 900.degree. C. at the nucleation zone in order to bring about the desired reaction. Various inert carrier gases such as, for example, hydrogen can be used to carry the dopant or thin film to the substrate in a vapor form.
Carrier gases are typically used to carry a source (either a solid, gas or liquid source) to the reactor or reaction chamber. The device used to inject a stream of carrier gas into a liquid source is generally referred to as a "bubbler." A typical bubbler is described and shown in U.S. Pat. No. 5,078,922 to Collins, et al. (herein incorporated by reference).
Reactions can take place in either pressurized or non-pressurized CVD chambers. Due to the stringent requirements of film uniformity, low pressure chemical vapor deposition (LPCVD) reactors have gained in popularity. LPCVDs generally operate in the range of 0.1 to 10 torr and can provide high quality film upon the substrate with relatively few impurities or contaminants therein. In many instances, film uniformity or impurity concentration is not critical. In those cases, atmospheric pressure chemical vapor deposition (APCVD) reactors are employed. APCVDs operate at atmospheric pressure and provide higher film growth rate than LPCVDs. Thus, APCVDs can increase wafer throughput.
More recent enhancements to APCVD systems include continuous APCVD systems. Continuous APCVD systems generally utilize a conveyor belt extending through multiple reactors arranged along the belt, whereby wafers are loaded onto the belt and pass through the reactors. See, e.g., Lee, H., Fundamentals of Microelectronics Processing, McGraw-Hill, 1990, pp. 220-281. Continuous APCVD systems are well suited for depositing films such as glass or BPSG which do not require stringent flatness or uniformity upon the substrate.
Although continuous APCVD systems are well recognized for their throughput feature, they must be occasionally cleaned of the deposits left behind. Periodically, the deposition material must be removed from the accumulation points within or around each reactor along the system. Specifically, the conveyor belt and drive systems must be periodically cleaned as well as nitrogen purge curtains surrounding each reactor. Moreover, the muffle surrounding the reactions chambers and, particularly, perforations within the muffle directly below each reaction site must be cleaned to ensure the openings remain. If the reaction area and regions surrounding the reaction area (i.e., purge curtains, floor purge perforations, conveyor belt, etc.) are not periodically cleaned of deposits, proper amounts of nitrogen purge will not occur. As a result, the viscous flow from the purge curtains may not occur thereby causing deleterious gas phase nucleation (i.e., nucleation in the gas or vapor surrounding the wafer instead of on the wafer itself). If the perforations underneath each reaction area are filled or become plugged with deposits, proper floor purge through the perforations will not occur. Once the perforations underneath each reactor (i.e., underneath the conveyor belt and wafer substrate) become plugged, nitrogen flow from underneath the reaction site and through the holes will cease thereby allowing impurities to creep into the chamber and collect upon the backside of the wafer. It is important that viscous flow with closely controlled and equalized flow rates occur and are properly directed in and around each of the multiple reactor sites of the continuous APCVD system. Any change or fluctuation from optimal flow rates or redirection of purge gases generally caused by deposit buildup will negatively impact the deposition operation.
It is not only important to periodically clean the reaction areas, but the cleaning step must be non-intrusive and must be easily performed. It is further important that each reaction area be cleaned thoroughly such that the continuous APCVD system, having many reaction areas, is completely cleaned throughout. Each reaction area or chamber must be cleaned simultaneously and at the same rate. If one reaction area is cleaned at a faster rate than another, then one reactor of the continuous APCVD system may be left dirty while the remaining reactors are clean. Additionally, over cleaning of one reactor of the plurality of reactors may cause problems whenever the etching material used to clean the reactor over etches into the reactor material itself instead of simply removing only the deposits upon the reactor surface.
It is therefore important that a cleaning apparatus and method be devised which can clean several reactors or reaction areas simultaneously within a continuous APCVD system. Likewise, it is necessary that the cleaning material used to remove the deposits be directed to areas where deposits are predominately placed and areas where deposits cannot be readily tolerated. For example, it is important that the cleaning material be directed to any and all deposit sites along the continuous APCVD system with minimal operator interface using the existing CVD configuration.